1. Field of the Invention
The present invention relates generally to a balanced differential amplifier applied to a read amplifier for amplifying a read signal supplied from a storage device incorporated with a computer, and more specifically to an improvement of the overall constant current characteristics of the balanced differential amplifier.
2. Description of the Prior Art
Conventionally, a read amplifier, i.e. a sensing amplifier, is used to amplify a minute read signal supplied from an inner storage device or an outer storage device, i.e. a memory unit, of an electronic computer to a voltage level sufficiently high to activate logic circuits. In the read amplifier, since suppression of two in-phase input signals is required in addition to linear amplification, the circuit is basically configured to depend upon differential amplifiers.
The balanced output signal obtained from the output terminals of a differential amplifier is usually supplied to a succeeding-stage amplifier. In a read amplifier for a high-speed multi-output memory unit, however, since the succeeding-stage amplifier usually has a large driving capability, when a large capacitive load is connected to the output terminals of the succeeding-stage differential amplifier, a large-amplitude transient current flows through the succeeding-stage amplifier to the power supply or the ground. Therefore, high-frequency noise is readily generated and enters into the differential amplifier in phase through the power supply or the ground. When the noise is inputted in phase to the differential amplifier, it is of course expected that the noise is not amplified but rather is cancelled out.
However, since the differential amplifier is normally provided with a constant current source which operates in an unsaturated region, the magnitude of current flowing through the constant current source is subjected to the influence of the absolute value of the read signal DC voltage supplied to the input terminals of the differential amplifiers. Thus, the constant current characteristics are deteriorated.
Therefore, in the prior-art differential amplifier there exists a problem in that the in-phase noise is not perfectly suppressed due to fluctuations of the constant current and, therefore, a noise component is still superposed upon the output signal of the differential amplifier.
The structure and operation of the prior-art balanced differential amplifier will be described in further detail hereinafter with reference to the attached drawings in the detailed description of the preferred embodiment which follows.